1. Field of the Invention
The present invention relates to an improvement of a symmetrical magnetic field type objective lens destined for use in an electron microscope.
2. Description of the Prior Art
In general, a spherical aberration coefficient (C.sub.s) and a chromatic aberration coefficient (C.sub.c) provide important factors in determination of performance of the objective lens for the electron microscope. The objective lens which suffers such aberrations to lesser degree is considered to exhibit an improved performance. Further, in order to realize an objective lens having a satisfactory performance for the electron microscope, it is required that a wide field of view can be obtained and that neither blurs nor distortions occur in a peripheral portion of the field of view. There has hitherto been known no objective lenses for the microscope which satisfy the requirements described above, presenting problems that the various aberrations of the objective lens make appearance noticeably and/or difficulty is encountered in attaining a wide field of view, particularly when image of a specimen is observed with a small magnification.
As an attempt to reduce the adverse influences ascribable to the spherical aberration and the chromatic aberration, there is already known an objective lens which is referred to as Riecke - Ruska's condenser objective lens and in which the aberration coefficients described above are reduced by increasing magnetic excitation for the objective lens. Hereinafter, the magnetomotive force of the Riecke-Ruska's condenser objective lens will be represented by Jc. o. Since this condenser objective lens is also of a symmetrical magnetic field type, apertures formed in an upper magnetic pole piece and a lower magnetic pole piece are imparted with a same diameter, wherein a specimen to be observed is positioned at a middle point between the upper and the lower magnetic pole pieces, as will be described hereinafter in more detail in conjunction with FIG. 1 of the accompanying drawings. However, because more intensive excitation is effected for a magnetic field at the side of a condenser lens than at the location of a specimen in the case of the electron microscope incorporating the condenser objective lens, resulting in a correspondingly intensified convergence of the electron beam through the excitation, difficulty is encountered in the control of the illuminating electron beam. Further, it is difficult to attain a wide field of view in the observation with a small magnification, to a further disadvantage.
On the other hand, in the case of a conventional electron microscope in which no use is made of the condenser objective lens, the objective lens is excited with a smaller magnetomotive force than that (Jc. o.) of the condenser objective lens. As a result, the spherical aberration coefficient as well as the chromatic aberration coefficient tends to be increased to inconvenience. When the magnetomotive force is increased up to the level corresponding to Jc. o. in an effort to reduce these aberration coefficients, then there will arise a problem that an image of a high quality is difficult to be produced at a low magnification as in the case of the condenser objective lens.
As the objective lens having the magnetomotive force of a greater magnitude than the one corresponding to Jc.o., there is known Suzuki-Tochigi's S-zone lens. In the electron microscope using this lens, the specimen is disposed at a position lower than the middle or center point between the upper and the lower magnetic pole pieces. The electron flux incident in parallel onto the objective lens is once converged previously and impinges on the specimen in parallel. With the S-zone lens, it is certainly possible to reduce the aberration coefficients more than the condenser objective lens, so far as the excitation of the magnetic pole pieces remains unsaturated, while the irradiating electron beam can be controlled easily as is in the case of the objective lens whose magnetomotive force is smaller than that of the condenser objective lens. However, difficulty is also involved in the observation with a low magnification in respect that the field of view is in practice decreased due to an objective aperture and/or remarkable distortion occurs in the produced image.